1. Field of the Invention
The present invention relates to fixed-type constant velocity universal joints. Constant velocity universal joints are used in the power transmission of automobiles and various industrial machines and transmit torque from a driving shaft to a driven shaft at a constant angular velocity. Two types of constant velocity universal joints are known: fixed-type, which permits only angular displacement, and slidable-type, which permits both angular and axial displacement.
2. Description of the Related Art
A typical fixed-type constant velocity universal joint includes the following components: an outer joint member that is connected to a driving or driven shaft in a torque-transmitting manner; an inner joint member that is connected to the driven or driving shaft in a torque-transmitting manner; a plurality of balls placed between the outer and inner joint members to transmit torque; and a cage that holds the balls within the bisecting-plane of the angle between the driving and driven shafts.
Recent concern about ensuring safety of automobiles upon collision has led to the design of automobiles with longer wheelbase. To prevent the associated increase in the steering radius, the steering angle of the front wheels needs to be increased by increasing the operating angle of the fixed-type constant velocity universal joints, which are used as connecting joints in the drive shafts of automobiles.
To increase the operating angle of fixed-type constant velocity universal joints, it has been proposed to form track grooves with tapered bottoms in the outer and joint members so that the corresponding grooves together form ball tracks in which the balls roll (See, for example, Japanese Patent Laid-Open Publications No. 2001-153149, No. 2001-304282, and No. 2001-349332).
In previous attempts to increase the operating angle of fixed-type constant velocity universal joints, defining the taper angle of the track grooves of the outer and inner joint members has not been considered particularly important.
Although the fixed-type constant velocity universal joints described in the above-mentioned publications each have outer and inner joint members featuring tapered track grooves that permit a larger operating angle of the joint, the current trend toward lightweight and compact joints puts a limitation on the outer diameter of the outer joint member, and if the taper angle of the track grooves is to be increased, the wall thickness of the outer joint member will inevitably be decreased, resulting in a decreased strength of the outer joint member. The increase in the taper angle of the track grooves also results in an increase in the load exerted upon the side wall of the cage pocket by the balls received by the cage: The load exerted upon the cage pocket increases in proportion to the taper angle of the track groove (FIG. 22).
In the fixed-type constant velocity universal joint disclosed in Japanese Patent Laid-Open Publication No. 2001-153149, the amount of the cage offset is large enough so that the load exerted upon the cage pocket by the ball in its outermost position or phase (phase angle φ=0°; FIG. 3) can be kept small. In other conventional constant velocity universal joints, however, the load exerted by the ball in the 0° phase generally increases as the operating angle of the joint is increased.